1. Field of the Invention
The present invention relates to plasma processing and more particularly to a system including a gas injection component for improved plasma processing.
2. Discussion of the Background
Typically, during materials processing, plasma is employed to facilitate the addition and removal of material films when fabricating composite material structures. For example, in semiconductor processing, a (dry) plasma etch process is utilized to remove or etch material along fine lines or within vias or contacts patterned on a silicon substrate. The plasma etch process generally involves positioning a semiconductor substrate with an overlying patterned, protective layer (for example, a photoresist layer) into a processing chamber. Once the substrate is positioned within the chamber, an ionizable, dissociative gas mixture is introduced within the chamber at a pre-specified flow rate, while a vacuum pump is throttled to achieve an ambient process pressure.
Thereafter, a plasma is formed when a fraction of the gas species present are ionized by electrons heated via the transfer of radio frequency (RF) power either inductively or capacitively, or microwave power using, for example, electron cyclotron resonance (ECR). Moreover, the heated electrons serve to dissociate some species of the ambient gas species and create reactant specie(s) suitable for the exposed surface etch chemistry. Once the plasma is formed, any exposed surfaces of the substrate are etched by the plasma. The process is adjusted to achieve optimal conditions, including an appropriate concentration of desirable reactant and ion populations to etch various features (e.g., trenches, vias, contacts, etc.) in the exposed regions of the substrate. Such substrate materials where etching is required include silicon dioxide (SiO2), poly-silicon and silicon nitride.
As the feature size shrinks and the number and complexity of the etch process steps used during integrated circuit (IC) fabrication escalate, the ability to control the transport of reactive materials to and effluent etch products from etch features becomes more stringent. The ability to control such processes must insure achievement of the proper chemical balance necessary to attain high etch rates with good material selectivity.
The etch rate in most dry etch applications (for example, oxide (SiO2) etch applications) includes a physical component and a chemical component. The plasma chemistry should create a population of positively charged (for example, relatively heavy, singly charged argon ions) utilized as the physical component and a population of chemical radicals (such as atomic fluorine F, and CF, CF2, CF3 or more generally CFx species in a fluorocarbon plasma) utilized for the chemical component. Moreover, the chemical reactants (CFx) act as reactants in the surface etch chemistry and the (heavy) positively charged ions (Ar+) provide energy to catalyze the surface reactions.
As feature sizes progressively shrink, they do so at a rate greater than the shrinking oxide and other film thicknesses. Therefore, the etch feature aspect ratio (depth-to-width) is greatly increased with shrinking sizes, for example on the order 10:1. As the aspect ratio increases, the directionality of chemical reactant and ion transport local to the etch features becomes increasingly important in order to preserve the anisotropy of the etch feature.
The transport of electrically charged species such as ions can be affected by an electric force and, therefore, it is conventional in the industry to provide a substrate holder or chuck with an RF bias to attract and accelerate ions to the substrate surface through the plasma sheath such that they arrive moving in a direction substantially normal to the substrate surface. However, the transport of neutral, chemically reactive species is not amenable to the application of an electric force to assert their directionality at the substrate surface. In order to affect the same, gas species are injected under high pressure to preserve their directionality upon expanding into the low-pressure environment and the ambient pressure is sufficiently reduced to further maintain a narrow angular distribution of the velocity field near the substrate surface.
In order to affect changes in the transport of neutral species local to the substrate surface and, thus, affect the etch rate of high aspect ratio features, the gas injection total pressure can be increased. One way to increase the gas injection total pressure is to increase the mass flow rate of process gas into the process chamber. However, in order to achieve the same process pressure, the pumping speed to the processing region must be proportionately increased. For example, when utilizing a shower-head gas injection system including approximately three-hundred-and-sixty 0.5 mm diameter orifices of 1 cm in length with a typical process gas flow rate, for example, of 400 sccm (standard cubic centimeters per minute) argon, the gas injection total pressure can be approximately 6 Torr. In order to increase the gas injection total pressure by a factor of ten (e.g., from 6 to 60 Torr), the mass flow rate must be increased by a factor of ten (e.g., from 400 to 4000 sccm argon) and, therefore to achieve the same process pressure of, for example, 20 mTorr, the pumping speed at the processing region must also be increased by a factor of ten (e.g., from 250 to 2500 liters per second).
A turbo-molecular pump (TMP) with a minimum inlet pumping speed of 5000 liters per second is employed to achieve this demand for pumping speed. The TMP includes a vacuum chamber configured to have at least a flow conductance equivalent to that of the inlet pumping speed to the TMP. However, these high-speed pumps are cumbersome, and extremely expensive (e.g., currently such pumps cost approximately $100,000 per pump with an additional $50,000 for an appropriately sized gate valve). Moreover, to accommodate their size and enable the aforesaid flow conductance, the process chamber footprint must be increased. Consequently, they are not an economically viable solution. Therefore, what is needed is a way to substantially increase the gas injection total pressure while satisfying an upper limit to the process gas flow rate.
Another technique is to pulse the gas in short bursts as it enters the process chamber. However, this technique results in transient pressure waves due to the large volume of gas introduced to the chamber with each pulse. In particular, there is an abrupt increase in chamber pressure followed by a slow decay as the vacuum pump struggles to bring the pressure back to specification. What is needed is a way to increase the gas injection total pressure without creating a substantially non-stationary process.
A further method of increasing the gas injection total pressure, while not affecting the mass flow rate or process chamber pressure, involves reducing the number and/or size of the gas injection orifices (i.e. reducing the total injection flow-through area). However, this reduction can seriously jeopardize process uniformity over large substrates, for example, 200 to 300 mm and greater. Conventional systems achieve uniform etching of substrates by maintaining a uniform gas flow over the substrate surface (in addition to other requirements). In order to achieve a uniform introduction of the process gas to the process environment, materials processing devices utilize a showerhead gas distribution system comprising a plurality of gas orifices, for example, on the order 100 to 1000 gas injection orifices of 0.5 mm in diameter. However, to maintain conventional process recipes (i.e. chamber pressure and gas flow rate) optimal for etch applications such as oxide etch applications, the injection pressure is limited and neutral flow directionality suitable for high aspect ratio feature etch is sacrificed. What is needed is a way to increase the gas injection total pressure while achieving specifications for process uniformity and employing practical gas injection orifice geometry.